An electromechanical power transmission chain for driving an actuator, e.g. a wheel or a chain track, comprises typically a capacitive circuit, one or more electric machines, and converter equipment for transferring electric energy between the capacitive circuit and the one or more electric machines. The electromechanical power transmission chain can be a series transmission chain where at least one of the electrical machines operates as a generator and the converter equipment comprises one or more converter stages for transferring electric energy from each generator to the capacitive circuit and one or more other converter stages for transferring electric energy from the capacitive circuit to each electric machine which acts as an electric motor for driving an actuator. Each generator can be e.g. an electrically excited synchronous generator or a permanent magnet synchronous generator and the converter stage between the generator and the capacitive circuit can be for example a pulse width modulation “PWM” converter stage. Each electric motor can be e.g. a permanent magnet or induction motor and the converter stage between the capacitive circuit and the electric motor can be e.g. a PWM-converter stage. Each generator can be driven with a combustion engine that can be e.g. a diesel engine, an Otto-cycle engine, or a turbine engine.
An electromechanical power transmission chain can be as well a parallel transmission chain where an electric machine is mechanically connected to a combustion engine and also to an actuator. The electric machine operates sometimes as a generator which charges the capacitive circuit and/or another energy-storage of the electromechanical power transmission chain, and sometimes as an electric motor that receives electric energy from the capacitive circuit and/or the other energy-storage and assists the combustion engine when high mechanical output power is needed. It is also possible that an electromechanical power transmission chain is a combined series-parallel transmission chain so that one or more electric machines are mechanically connected to both a combustion engine and an actuator and one more other electrical machines are arranged to drive one or more other actuators in the same way as in a series transmission chain.
Electromechanical power transmission chains of the kind mentioned above provide advantages compared to a traditional mechanical power transmission chain because, for example, the rotational speed-torque operating point of the combustion engine can be more freely selected from the viewpoint of the operational efficiency of the combustion engine, and thus savings in the fuel costs can be achieved. In many cases, this advantage is achieved so that the above-mentioned capacitive circuit is charged when only low mechanical output power is needed and discharged when high mechanical output power is needed. In other words, the capacitive circuit is used as an energy buffer. The use of the capacitive circuit as an energy buffer is, however, not free from challenges. The electrical energy stored by the capacitive circuit is directly proportional to the square of the voltage of the capacitive circuit, and thus the direct voltage of the capacitive circuit varies when the capacitive circuit acts as an energy buffer. The variation of the direct voltage complicates the control of the electric machines of the electromechanical power transmission chain. Furthermore, in situations where the above-mentioned direct voltage is low, the magnetic fluxes in the electric machines can be so small that the operating efficiencies and maximum torques of the electric machines are decreased.